The present application claims priority from European Patent Application No. 02396036.2, filed Mar. 22, 2002.
The invention relates to a gas analyzer comprising: a measuring volume having connections for input and output flow of a sample gas mixture, at least one gas component of which is to be analyzed for determining its concentration in said mixture, and having first and second ends transparent to radiation; a radiation source for providing a beam of electromagnetic radiation having a wavelength range, said beam approaching collimation and being directed to pass said measuring volume through the first and second ends thereof; a heat sink for said radiation source; at least one thermal detector having an active radiation detecting sensor element within at least one detector housing and receiving the radiation exiting said measuring volume, said thermal detector having a reference sensor element within the same detector housing and protected from said radiation, said thermal detector generating an output signal indicative of a property of said at least one gas component of said mixture in the measuring volume; at least one optical bandpass filter between said active radiation detecting sensor element and said radiation source; electrical contact pins in said at least one detector housing for the output of said signal(s); a thermal mass formed of a material having high thermal conductance, said thermal mass: having a cavity and an outer surface, surrounding at least said detector housing in the cavity, being in contact with said detector housing, and extending towards the radiation source; and a thermal barrier between the heat sink and the thermal mass.
Thermal detectors, typically thermopile detectors are used in gas analyzers among other things owing to their capability of DC (Direct Current) measurement, which facilitates a cost-effective construction of the measurement system. In these gas analyzers the thermopile detector measures the infrared absorption of a gas introduced to a sampling chamber or a sampling volume, after which the concentration of the gas component or the gas components of interest is/are determined from the measured absorption. The useful wavelength range of thermopile detectors is suitable for infrared measurements, since their absorption bands in the wavelength region 3 xcexcm-10 xcexcm fall within the required spectral sensitivity wavelength range for the detector. Moreover, thermopiles have a high sensitivity and good linearity and they are cost effective components.
A characteristic of a thermopile detector is that a thermal gradient in its external housing, noticeable especially in small analyzers with small thermal mass, will cause an offset error in the detector signal, which degrades measurement accuracy. The thermopile is a very sensitive detector containing a plurality of thermocouple junctions. In a typical analyzer it has been measured that the signal change corresponding to the absorption caused by 0.1% by volume of CO2 in a sample gas is about 2 xcexcV. The temperature difference in the thermopile detector would then be only about 0.13 mK. It is therefore easy to understand that even small temperature gradients in the thermopile housing may cause considerable measurement errors. Similar errors also occur with a change of the external housing temperature after, e.g., a cold start-up of the analyzer or due to a change in the ambient temperature.
With reference to the patent U.S. Pat. No. 4,772,790 a gas analyzer is described where a number of thermocouples connected to form groups of thermopiles are used as the detector. The first set of thermopile groups is arranged so as to receive the radiation at their inner junctions to form hot junctions, whereas the outer junctions shielded accordingly form cold junctions of this first set. The thermocouples of the second set of thermopile groups, having the same number of shielded thermocouples as the first set, are electrically connected in series with the first set, whereupon the electrical current caused by the EMFs from the first set because of the radiation creates inverted cold as well as hot junctions in the second set of thermopile groups with opposite EMFs to those of the thermocouples of the first set. All of the thermocouples with their hot and cold junctions are positioned on a single substrate of a heat-conductive insulator material. The analyzer is further provided with a highly heat conductive section, which is in contact with the ambience and has a thermal mass substantially greater than that of the housing so that it acts as a large area heat sink. The heat-conductive substrate of the thermopiles is connected with the conductive section using a heat conductive material. Further the radiation source is thermally insulated from the heat conductive section by the wall of the sample cell made of plastic or the like. This seems to be the conventional compensation method. In this construction there is a need for additional dark junctions, which reduces the space for the sensitive area of the detector. The leads from the detector housing are directly connected to the conductive pads of a printed circuit board in contact with ambience, too. As a consequence, even an extremely small change of the temperature from the ambience would cause a considerable thermal gradient on the substrate and so within the thermopile array. Especially in a small analyzer such a gradient could induce an offset in the signal, which would not necessarily be completely compensated by the shielded junctions. For modern semiconductor thermopiles bonded to the base plate of the housing this is especially true. Under a steady state condition, in which the non-shielded hot junctions are receiving a constant radiation, there may exist even a temporal thermal drift, whereupon the DC-signal from the detector varies with time, leading to measurement inaccuracies.
The patent U.S. Pat. No. 5,081,998 discloses a gas analyzer where a group of thermocouples is connected in series and paired so that the first thermopile and the second thermopile is in opposed relationship to each other on a common ceramic substrate. The first thermopile is electrically connected to the second thermopile in series opposition to subtract the output signals from each other, and further at least a first neutral density, i.e. attenuation filter with a different transmission coefficient as compared to a second or lacking neutral density filter are positioned in front of the thermopile detectors so that the first neutral density filter affects the hot junctions of the first thermopile and the second neutral density filter or its absence affects the hot junctions of the second thermopile, whereupon these two thermopile detectors are xe2x80x9coptically stabilizedxe2x80x9d. The thermopile detectors are further preceded by one or several analytical bandpass filters and a reference bandpass filter for passing desired wavelengths in the optical path. The difference between the outputs is used to eliminate the effects of a variation in the background signals and variations due to the thermal drift. This kind of construction makes the detector large and expensive and also difficult to construct for several different gases.
The patent U.S. Pat. No. 5,296,706 refers e.g. to those two patent publications mentioned above, and also describes, as a prior art, a further developed version of the latter patent provided a multiple of paired thermopiles and with an aperture sheet placed over the optical filters for analysis of several gas components in the gas mixture. This now discussed US-patent discloses a topography, which allows several channels to be used as independent analytical channels for detecting the absorption of a plurality of predetermined wavelengths. For this purpose the patent suggests separate reference thermopiles, which are identical with the active analytical thermopiles and are located behind these active thermopiles receiving the thermal radiation. Each reference thermopile and its corresponding active thermopile are disposed back to back in xe2x80x9cparallel opposedxe2x80x9d fashion with shielding means such as aluminum foil therebetween, whereupon the reference thermopiles are protected from all incident radiation. Each reference thermopile produces a signal representative of ambient temperature transients, the corresponding active thermopile produces a signal representative of the received radiation and ambient temperature transients, and these signals are processed to produce a combination signal with an intention that the effect of said thermal transients were eliminated, unlike in the arrangement of the U.S. Pat. No. 5,081,998. This kind of analyzer can be constructed for analysis of several gases but the thermopile structure is very complicated and expensive. Additionally the suggested calculations using a second order polynomial equation having cross product terms to calibration and compensation is considered to be a complicated task. Thermal gradients may also affect the thermopiles of the pair differently thus still inducing an offset in the signal. The patent U.S. Pat. No. 6,277,081 refers to the last mentioned two patent publications, too. Aiming to analyze carbon dioxide and more than five further anesthetic gas components, the suggested apparatus comprises a plurality of independent detectors provided with optical filters having particular wavelength transmissions for different gases, and also a plurality of reference detectors provided with further optical filters having particular wavelength transmissions for optical references. Additionally, the apparatus comprises at least one detector with an opaque optical filter to prevent substantially all radiation from reaching this blocked detector, and the number of these blocked detectors is fewer than said independent detectors. A large DC offset voltage is described as typical for all the detectors in an analyzer in the absence of any infrared radiation, and a specific mathematical compensation is applied after calibration. No attempt has been made to minimize said offset. As said xe2x80x9cdarkxe2x80x9d offset signal is described to be typically 2 to 6 times the measurement signal received, the offset certainly has considerable influence on the reliability of the sensor between calibration events and during start-up.
Further the patent U.S. Pat. No. 5,542,285 discloses different means for compensating the thermal errors by describing a gas analysis apparatus in which compensation is provided for transient errors caused by temperature changes associated with said apparatus, said apparatus comprising: a sample cell containing the gas to be analyzed; an electromagnetic radiation source for passing electromagnetic radiation through the sample cell; a thermal detector having a radiation detecting sensor element receiving the radiation exiting said sample cell, said thermal detector having a reference sensor element protected from said radiation, said thermal detector generating an output signal indicative of a property of the gas in the cell, changes in the temperature of the thermal detector introducing errors in the output signal; temperature sensing means providing a temperature signal indicative of the temperature of the reference sensor element; and signal processing means coupled to said sensing means for determining the rate of change of temperature of said reference sensor element as a function of time and providing a compensating signal indicative of same, said signal processing means being coupled to said thermal detector and for altering the output signal of said thermal detector in accordance with the compensating signal to provide a temperature compensated output signal indicative of the property of the gas in said cell.
As to the measuring principle of the analyzers described above, it should be noticed that the actual target for measurement is the gas mixture, more specifically some of the gas components with variable concentrations and their absorptions, which are variable accordingly. The concentrations, not temperatures are the object for the measurement. The radiation sources are neither the target nor the object for measurement, because they should not be variable, but the temperature of these IR-sources shall be as constant as possible.
The patent U.S. Pat. No. 5,012,813 discloses a tympanic temperature measurement device to provide accuracy within one-tenth of a degree over limited ranges of ambient temperature and accuracy to within one degree over a wide range of ambient temperatures. The radiation detector for detecting the temperature of the tympanic membrane area at about the body temperature of a patient according to the patent comprises: a thermopile having a hot junction and a cold junction, the hot junction being mounted to view a target source; a temperature sensor for sensing the temperature of the cold junction; an electronic circuit coupled to the thermopile and responsive to the voltage across the thermopile and a temperature sensed by the temperature sensor to determine the temperature of the target, the electronic circuit determining the temperature of the target as a function of the voltage across the thermopile and the temperature of the hot junction of the thermopile determined from the cold junction temperature and a thermopile coefficient; and a display for displaying an indication of the temperature of the target determined by the electronic circuit. The electronic circuit determines the temperature of the target source from the relationship TT4=(Khxc3x97H)+TH4 where TT is the target temperature, Kh is a gain factor, H is a sensed voltage from the thermopile and TH is the hot junction temperature of the thermopile. Further according to the patent, the thermopile is positioned within a rear volume in a can of high conductivity material. The can comprises a radiation guide with a tapered form and with a germanium window at the front end, an additional conductive thermal mass surrounding the can and said rear volume, and a conductive plug attached to the rear end of the thermal mass and also surrounding the volume. The can is filled with a gas of low thermal conductivity such as Xenon surrounding the thermopile. The radiation guide is formed of a single piece of high conductivity material such as copper. Both the additional conductive thermal mass and the conductive plug are of a high conductivity material such as copper, too, and they are in close thermal contact with the can and with each other. According to the patent the output of the thermopile is a function of the difference in temperature between its warm junction, heated only by radiation viewed through the window, and its cold junction, which is in close thermal contact with the can, whereupon the radiation guide should be, throughout a measurement, at the same temperature as the cold junction. The patent does not describe how this close thermal contact with the can is achieved; the thermopile seems to be without any contacts in the center of said rear volume. The temperature of the cold junction is anyway monitored by a separate thermistor positioned within the conductive plug, and the signal voltage from the thermopile corrected respectively. Disclosed is that to minimize the temperature changes, the radiation guide and the can are well insulated by means of a casing of plastic material having low thermal conductivity and an insulating air space, but a high conductance thermal path is provided between the foremost end of the radiation guide and the portion of the can surrounding the thermopile to distribute any changes in temperature rapidly to the cold junction to avoid thermal gradients. This high conductance of the thermal path is enhanced by the unitary construction eliminating any thermal barriers. So, here the radiation source itself being independent from the measuring apparatus is the target, the temperature of which is measured as the object.
Further developments to the probe of this patent mentioned above is disclosed in patent U.S. Pat. No. 5,445,158. Here too, the thermal mass is of unitary construction which eliminates thermal barriers between the tube and the portion of the thermal mass surrounding the thermopile, and a plug of high thermal conductivity material positioned behind the thermopile is in close thermal contact with the mass. The outer sleeve is formed of low thermal conductivity plastic and is separated from the mass by an insulating air space. The taper of the mass increases the insulating air space adjacent to the end of the extension while minimizing thermal resistance from the tube to the thermopile. The rings, the window and the header are thermally coupled by high thermal conductivity epoxy. This way the thermal RC time constant for thermal conduction through the thermal barrier to the thermal mass and tube is at least two orders of magnitude greater than the thermal RC time constant for the temperature response of the cold junction to heat transferred to the tube and thermal mass. The RC time constant for conduction through the thermal barrier is made large by the large thermal resistance through the thermal barrier and by the large thermal capacitance of the thermal mass. The RC time constant for response of the cold junction is made low by the low resistance path to the cold junction through the highly conductive thermal mass, and the low thermal capacitance of the stack of beryllium oxide rings to the thermopile. Besides the transfer of heat from the environment, another significant heat flow path in the system is through the leads. To minimize heat transfer through that path, the lead diameters are kept small and the leads are trimmed off in the region. A pair of 40 gauge wiresxe2x80x940.079 mm diameter, respective cross sectional area of 0.0049 mm2xe2x80x94are soldered to the shortened leads formed of 20 mils of kovar providing structural support to the thermopile assembly. The wires extend from the region through the plug and conduct thermopile signals to the electronics. Further potential heat flow path in the system is through the header to the plug. Since the header is in close thermal contact with the thermopile cold junction, any thermal gradients through the header would be amplified 100 to 1000 times by the thermopile producing large error signals. To eliminate the same an insulating region of air is provided behind the header to heat transfer through that path. Thus, any thermal gradients in the plug would be forced to travel through the mass and would be substantially dissipated without affecting the thermopile. In addition to the germanium window at the front end of the probe, two different embodiments with either a silicon window or no window at the front end of the probe are disclosed.
The common general feature of the probes in accordance with both U.S. Pat. No. 5,012,813 and U.S. Pat. No. 5,445,158 is that radiation is measured from the tympanic membrane area at about body temperature. The wavelength region is equivalent to that of the transmission of germanium, about 1.8-23 xcexcm, or to that of the transmission of silicon, about 1.1-40 xcexcm, or without any limits in embodiments having no window. This gives a signal that well reflects the blackbody radiation or respective temperature of the patient""s body. The amount of radiation is small but the very broad wavelength region still gives a fair signal, especially with the special elongated thermally conducting tube around the thermopile.
It is an object of the present invention to overcome the shortcomings of the above described prior-art techniques and to achieve a novel type of non-dispersive gas analyzer for eliminating offset and drift caused by thermal gradients in the analyzer. Especially it is an object of the present invention to eliminate the large offset signal caused by thermal radiation source induced static temperature gradient along the analyzer body not present in the temperature sensor without an internal source. Further it is an object of the present invention to achieve a small sized or miniature gas analyzer, which fulfills these objects, and in which also economic commercially available thermopile detectors may be used.
The invention is based on eliminating thermal offset and drift by minimizing the thermal gradients over the complete detector housing, including its electrical connections. This is attained by a gas analyzer in which the electrical wires are composed of materials and have dimensions producing an overall thermal conductance substantially lower than that of said electrical contact pins, the electrical wires are connected with the electrical contact pins either directly or indirectly, and enclosed in the thermal mass together with said detector housing(s), and the electrical wires are extending from the cavity through the thermal mass to the outside thereof with at least one exit point at said outer surface. Theoretically, there should not be any signal offset in a thermopile without radiation reaching its sensitive area. In order to achieve this there must not be any temperature difference between the hot junctions in the sensitive area and the cold reference junctions of the thermopile. This further means that no thermal gradient can be allowed within the detector housing in spite of the relatively high heat flow and small thermal mass of the small sized analyzer. There will always be a gradient from the analyzer to the ambient but according to the invention this gradient is transferred away from the detector housing and its electrical connections. This is done by completely enclosing the detector housing in a material with good thermal conductivity. Additionally, the thermal energy flow through the electrical connections are minimized in the invention. In gas analyzers radiation is provided by a fixed source and the wavelength region for measurement is narrow, typically less than 300 nm. This gives selectivity between the absorption of different gases fed to the sample volume of the analyzer. The absorption signal is typically small and proper design of the detector end of the analyzer is very important in order to achieve reliability and fast response, which is attained in the analyzer of the invention. The analyzer according to the invention has very small dimensions and weight so that the analyzer unit with radiation source, measuring volume and thermal detector(s) can be fitted directly on an ordinary printed circuit board. The construction according to the invention is such that it is possible to use simple commercial thermopile detectors. No special features like shielded or partly shielded detectors are needed. This makes the gas analyzer very cost effective and simple.
The gas analyzer of the invention can be used for, e.g., monitoring the composition of the airway gases of a patient anesthetized for the duration of an operation, whereupon the gases to be determined can include carbon dioxide (CO2), nitrous oxide (N2O) as well as at least one anesthetic agent.